1. Field of the Invention
The present invention relates to a projection lens set and particularly to a reflective projection lens set for a digital light processing (DLP) projector, which provides a high projection luminance and image quality, eliminates the problems caused by multi-mirror of image system, shortens the required length of the projection lens set and overcomes the short back focal length (BFL) problem.
2. Description of the Prior Art
A DLP projector comprises a Digital Mirror Device (DMD), for example manufactured by Texas Instrument Corporation, as a basic imaging device. Current commercial DLP projectors utilize transmission lenses to perform projection. FIG. 1 schematically shows a reverse telephoto lens set used in a current DLP. In this drawing, an optical axis was constructed by DMD 1, the first lens 2 and the second lens 3. The light transmitted by the DMD 1 goes through the first lens 2 and the second lens 3 to perform magnification.
The drawbacks of the foregoing reverse telephoto lens will be specified as follows.
First, since a lens with 100% light transmission is not achievable (for example, the transmission ratio of glass is 92%), the light transmitted by the DMD 1 is partly absorbed by the first and second lens. Thus, the projection luminance is decreased by approximately 20%. The quality of the projected image will be deteriorated when the illumination of the light source in the projector is not sufficient.
Second, since there are particular relationships between the object and image distance for projecting the image, a distance between the first and second lens is required. This makes it difficult to shorten the length of the reverse telephoto lenses along the optical axis, which opposes the trend towards lighter and smaller electronic products.
Third, referring to FIGS. 5A and 5B, the DMD is composed of a plurality of pixel mirrors 20. The pixel mirror 20 selectively reflects the incident light to the projection lens set 30 by varying its angle. The light reflected to the projection lens set 30 is called intended light; light not reflected to the projection lens set 30 is called xe2x88x9210xc2x0 unintended light. The xe2x88x9210xc2x0 unintended light will interfere with the intended light when not properly dealt with. Since xe2x88x9210xc2x0 unintended light is generated by DMD and will be magnified by the first lens and second lens during projection, the projected image quality of a DLP will suffer greatly.
Fourth, the first lens 2 and second lens 3 are positioned along and with their centers on the optical axis OP. The DMD 1 is positioned beneath the optical axis OP so that the projected image will be above the optical axis OP. The first lens 2 and second lens 3 should be aligned symmetrically to the optical axis. This constrains the first lens 2 and second lens 3 from being truncated on one side beneath or above the optical axis OP, and makes it difficult to reduce the cost.
The object of the present invention is to solve the above-mentioned problems and to provide a reflective projection lens set for a DLP projector, which provides a high projection luminance and image quality, eliminates the problems caused by multi-mirror of image system, shortens the required length between the projection lens set, overcomes the short back focal length (BFL) problem and precludes xe2x88x9210xc2x0 unintended light caused by DMD from interference.
Another object of this invention is to reduce the production cost. It utilizes the first mirror located beneath the optical axis and the second mirror located above the optical axis for projection. Thus, the parts of the first mirror above the optical axis and the second mirror beneath the optical axis may be truncated. The lens size could be diminished to save cost.
To achieve the above-mentioned objects, the reflective projection lens set for a digital light processing projector with a digital mirror device comprises a first and a second mirror. The first mirror has a reflective surface positioned to receive and therefore reflect the rays of intended light. The second mirror has a reflective surface positioned to receive and therefore reflect the rays of intended light reflected from the first mirror, and an opaque surface on its backside to prevent the rays of unintended light from intruding into the path of the rays of intended light.
In the foregoing reflective projection lens set, the first and the second mirror may be a combination of concave, convex, curved or paraboloid mirror.
In addition, the foregoing reflective projection lens set further comprises a lens module positioned in an optical path starting from the digital mirror device, turning at the first mirror and to the second mirror, whereby an image magnification or correction is carried out. The lens module may be arranged between the DMD and the first mirror, or the first and second mirror.
Moreover, the DMD is positioned beneath an optical axis and the rays of intended light re-reflected by the second mirror are projected above the optical axis. The parts of the first mirror above and the second mirror beneath the optical axis may be truncated.
Furthermore, in another embodiment of the present invention, the foregoing reflective projection lens set further comprises a third and fourth mirror. The third mirror has a reflective surface positioned to receive and therefore reflect the rays of intended light reflected from the second mirror. The fourth mirror has a reflective surface positioned to receive and therefore reflect the rays of intended light reflected from the third mirror, and an opaque surface on its backside to prevent the rays of unintended light from intruding into the path of the rays of intended light.